The masking level difference (MLD) is a psychoacoustic phenomenon in which binaurally masked hearing thresholds are improved by imposing an interaural phase disparity on the signal or on the masker. The masking level difference in a given test subject is dependent upon the ability of the binaural auditory system to correlate information present in both auditory channels and, it is known that the MLD is sensitive to lesions which disrupt normal phase information. Such lesions are Meniere's disease, lesion of the VIII nerve and brainstem lesions. Abnormal MLD's in the presence of normal sensitivity has been shown to be an especially sensitive indicator of retrocochlear lesions. This is discussed by Noffsinger et al, Acta Otolaryngol (Supp) p. 303 and by Olsen et al, Audiology, Vol. 15, pp. 287-301, 1976.
A technique for measuring MLD, which has gained widespread acceptance, is described by Olsen et al, supra. In this technique, masked hearing thresholds are measured for a pure tone of 500 Hz or spondee words under one homophasic and two antiphasic conditions. These conditions are: (1) Binaural homophasic in which a signal (denoted S) is in phase with itself at the two ears of the listener (the in-phase condition denoted by capital S.sub.o) and with a masker noise (denoted by N) in phase with itself at the two ears of the listener (the phase relation being denoted by N.sub.o); (2) Binaural antiphasic in which the signal is 180.degree. out of phase at the two ears of the listener (the phase relation being denoted by S.sub..pi.) and the masker noise being in-phase with itself at the two ears of the listener (N.sub.o); and (3) Binaural antiphasic with the signal in-phase (S.sub.o) and the masker noise 180.degree. out-of-phase (N.sub..pi.) at the two ears (S.sub.o N.sub..pi.). The tone signal is obtained by a Bekesy type audiometer operating in a standard pulsed mode of stimulus presentation. The masker noise is produced by a narrow-band noise generator which produces a 600 Hz band of noise centered at 500 Hz and set to an overall level of 80 dB. The tone signal and the masker noise are supplied to a network which allows mixing of the signal and noise in a pair of earphones and allows the phase reversal of either noise or the signal in one earphone. The subjects trace their thresholds by operating the audiometer switch in the conventional manner for at least one minute under each of the above-identified test conditions. Olsen, et al, supra, also describe a technique for measuring speech MLD for spondee words using one channel of a speech audiometer for the recorded speech signal and a masker of white noise.
In the conventional clinical audiometers, typically of the Bekesy type, the test signal is supplied from a source through a variable attenuator to either side of a pair of headphones. For testing hearing threshold for pure tones, the signal source is a variable frequency audio oscillator which is set at a fixed frequency for a given test. For measuring the hearing threshold for speech, the signal source is a recorded speech signal of certain words. The audiometer is provided with a noise channel which is used to supply a selected masker or noise signal to one side of the earphones at a prescribed intensity level. The test signal is initially supplied through the attenuator at a sub-threshold level and the test subject is provided with a control switch which when actuated in one direction causes the attenuation to decrease at a given time rate of change. When the subject first hears the signal, the switch is reversed and the attenuation is progressively increased until the subject is no longer able to hear the signal and reverses the switch again. This procedure is repeated for a period of time with automatic recording of the level at which attenuation is reversed, thereby providing data for determining the subject's hearing threshold. The audiometer is required to provide a wide range of power output, on the order of 100 decibels. At the lowest level of output, the signal voltage is a small fraction of a microvolt. It is of great importance that the signal be produced at the earphones without distortion. The noise level, such as thermal noise, must be negligible compared to the lowest level of signal voltage. This type of audiometer is described in the von Bekesy U.S. Pat. No. 2,563,384.
Others have devised arrangements for supplying the audiometer signal voltage and noise voltage outputs to the earphones with selected phase inversions. A known arrangement comprises a first mixer network connected between the audiometer signal voltage output and one side of the earphones and a second mixer network connected between the noise voltage output of the audiometer and the other side of the earphones. The signal voltage is applied to the first mixer network through a phase inverting switch and a transformer and the noise voltage output is applied directly to the first mixer. Similarly, the noise voltage output is supplied through a phase reversing switch and a transformer to the second mixer and the signal voltage is supplied directly to the second mixer. One difficulty with this arrangement is that it introduces a significant amount of attenuation between the audiometer output and the earphones.
A general objective of this invention is to provide an MLD adaptor for audiometers which overcomes the disadvantages of the prior art arrangements for MLD testing. Additional objectives will be set forth below.